Storage tanks for gasoline and other fuels at service stations are typically buried in the ground and have manifolds which open at ground level. These manifolds are connected to the interior of the storage tanks by fill pipes. These storage tanks are serviced by tanker trucks which have hoses that connect at their outer ends to the manifolds.
The fuel level in the storage tanks is typically monitored by the insertion of a wooden measuring stick or graduated pole through the fill pipe into the storage tank to the bottom thereof. Upon withdrawal of the measuring stick, the wet area of the stick will indicate the level of fuel in the storage tank.
Overfilling storage tanks with gasoline or similar fuels results in spillage of such fuels onto the ground which results in soil and air contamination and possibly other detrimental ecological and other consequences. It has been previously proposed to provide devices for closing the fill pipe of such storage tanks when filled to a preselected level of its maximum capacity. In order to prevent tampering and/or damage from accidental impacts or the like, such overfill prevention devices preferably are located entirely within the tank, in series with the fill pipe or "drop tube."
Retrofit installation of an overfill prevention device within a storage tank therefore requires that the device be sufficiently small, especially in its transverse dimension, as to be capable of passage through the tank's manifold and fill pipe. Some prior overfill prevention devices include at least one float that projects laterally from the main body of the device and undergoes arcuate pivotal movement in response to changes in the liquid level within the tank. Devices having the float and associated linkage components permanently projecting laterally from the housing of the device cannot be installed readily, if at all, within existing tanks since they will not pass through the tank's manifold and/or associated fill pipe. Even those devices having float and linkage components that deploy outwardly from the housing only after passage into the tank will not function properly, if at all, when a side wall or baffle of the tank intersects and obstructs the intended path of pivotal movement of the float. Additionally, and in all cases, the use of long interconnecting linkages between the floats and the components connected thereto significantly increases the cost of the overfill devices, and the risk of component malfunction.
Other overfill prevention devices, such as those disclosed in U.S. Pat. Nos. 5,095,937 and 5,141,019 to LeBlanc et al. and U.S. Pat. No. 5,010,915 to Johnson et al., use pairs of hollow tubular floats to provide two-stage float actuated shutoff. Unfortunately, such devices cannot be used for retrofit applications unless drop tubes having small transverse dimensions are employed therein since both the drop tube and surrounding tubular floats must fit within the fill pipe of the storage tank. The use of drop tubes having small transverse dimension is undesirable because they reduce the rate at which storage tanks can be filled and also complicate measurement of fuel levels therein using conventional gauge poles which must be carefully positioned to fit within the drop tube and overfill device without causing damage thereto.
Another undesirable feature of many overfill devices is that they have bleed ports which permit the introduction, albeit at a slow rate, of an unlimited amount of additional liquid into a tank following closure of the main control valve of the device. Such introduction through the bleed ports fosters overfilling by operators who attempt to dispense as much fuel as possible. Many overfill devices also permit the escape of hazardous vapors from the upper vapor filled portion of the tank to the atmosphere when the fill pipe cover is removed. Many regulatory bodies now prohibit the use of any devices which permit such vapors to escape into the atmosphere.
The overfill prevention device disclosed in the commonly assigned U.S. Pat. No. 5,027,870 to Butterfield, was a successful solution to many of the aforementioned deficiencies and disadvantages of prior overfill prevention devices. However, this prior overfill prevention device had certain disadvantages and deficiencies. Among these disadvantages and deficiencies were the premature unlatching and closure of the main valve by waves in the storage tank during filling and problems with the main valve returning to the fully open and latched position when the main valve float returned to its lower position. Still further, the main valve in the Butterfield overfill prevention device was susceptible to damage by the gauge pole or fuel level measuring stick upon insertion and/or removal thereof from the storage tank.